Chlor-alkali electrolysis involves the electrolysis of sodium chloride to form chlorine gas and sodium hydroxide (“caustic”). Chlor-alkali electrolysis is energy intensive, and the shift in Japan, for example, from a mercury or diaphragm process to an ion-exchange membrane process has reduced environmental problems and provided a tremendous savings in energy (about 40% savings over a period of about 25 years). However, even the most energy efficient, state-of-the-art membrane electrolyzer consumes around 2500 kilowatt-hours (kWh) of electricity to produce one ton of chlorine and additional power savings cannot be attained using the current process. For further energy savings, a drastic change is necessary using, for example, electrode reactions different from conventional ones. The use of gas diffusion electrodes employed in fuel cells may be the most feasible means to provide considerable power savings (See U.S. Pat. No. 6,117,286 to T. Shimamune et al. entitled “Electrolytic Cell Employing Gas Diffusion Electrode, which issued Sep. 12, 2000, incorporated by reference herein).
Gas diffusion electrodes enable a gaseous reactant to be easily fed to the electrodes of the cell. FIG. 1 shows a schematic representation of a configuration of a three-compartment type cell that employs a commonly used type of oxygen gas diffusion cathode. Cell 1 is divided by cation exchange membrane 2 into anode chamber 3 and cathode chamber 4. Cathode chamber 4 is divided by oxygen diffusion cathode 5 into solution chamber 6 and gas chamber 7. Oxygen gas as a starting material is fed from the gas chamber 7 side to the gas phase side of oxygen gas diffusion cathode 5. The oxygen gas diffuses through the oxygen gas diffusion cathode 5 and reacts with water (and electrons) in the catalyst layer of cathode 5 to generate sodium hydroxide. Gas diffusion cathode 5 is of the so-called gas/liquid separation type, which is permeable to oxygen and prevents sodium hydroxide from moving from solution chamber 6 to gas chamber 7. The oxygen gas diffusion cathodes that have been proposed so far as electrodes for chlor-alkali electrolysis satisfying this requirement are mostly gas diffusion electrodes produced by mixing carbon powder with PTFE, molding the mixture into a sheet to obtain an electrode base, and depositing a catalyst, e.g. silver or platinum, on the base.
In conventional sodium chloride electrolysis, the anodic and cathodic reactions are as follows:
Anodic reaction: 2Cl−→Cl2+2e (1.36 V).
Cathodic reaction: 2H2O+2e→4OH−+H2 (−0.83 V).
Thus, the theoretical thermodynamic decomposition voltage is 2.19 V.
When the above electrolysis is conducted while feeding oxygen to the cathode, the following cathodic reaction occurs:
Cathodic reaction: 2H2O+O2+4e→4OH− (0.40 V).
Thus, when an oxygen reduction reaction not involving hydrogen generation is used in place of the hydrogen generation reaction at the cathode in the conventional processes, the theoretical decomposition voltage decreases from 2.19 V (the conventional value) to 0.96 V. In theory, at least, a decrease in the decomposition voltage of 1.23 V (the difference between the thermodynamic potentials) is possible. In practice, however, the actual difference in cell voltage between hydrogen-evolving and oxygen-consuming cells can differ substantially from 1.23 V. This can be traced to the high temperatures (80–90° C.) required for chlor-alkali electrolysis, to differences in the kinetics of the hydrogen-evolving and oxygen-consuming chemical reactions, and also to differences in cell design. In any case, oxygen-consuming membrane cells offer a significant energy savings when compared to state-of-the-art hydrogen-evolving membrane cells. With this in mind, attempts have been undertaken worldwide to lower the energy consumption of chlor-alkali electrolysis by replacing a hydrogen-evolving cathode with a gas-diffusion type oxygen-consuming electrode.
For the three-compartment cell of FIG. 1, oxygen gas diffusion cathode 5 separates oxygen from sodium hydroxide; oxygen passes through one side of cathode 5 while sodium hydroxide generated exits the other side of cathode 5. A significant disadvantage of the three-compartment cell relates to the electrical energy loss resulting from the ohmic drop across the sodium hydroxide in between cathode 5 and the cation exchange membrane 2. The problem due to ohmic drop is minimized in a zero-gap electrolytic cell, a representation of which is shown in FIG. 2. In contrast to the cell of FIG. 1, zero-gap electrolytic cell 8 of FIG. 2 does not include the solution chamber in between the cathode and cation exchange membrane. Instead, zero-gap cell 8 includes an oxygen gas diffusion cathode 9 and an ion-exchange membrane 10 that are intimate contact with each other. Oxygen gas and water are fed as starting materials, and sodium hydroxide as a reaction product is recovered from the same side of cathode 9.
There are practical current density limits for membrane, hydrogen-evolving cells. Many hydrogen-evolving cells operate at an optimum current density of about 4 kA/m2, which provides a reasonable rate of generation of sodium hydroxide and chlorine gas, and with a high current efficiency. Oxygen-consuming membrane cells may also operate at this current density, but at a lower voltage, typically about 0.9 V lower than the voltage for a hydrogen-evolving membrane cell.
Oxygen gas diffusion cathodes suitable for use in a zero gap cell of the type shown in FIG. 2 should have high gas permeability, high hydrophobicity to avoid wetting by sodium hydroxide, and high permeability for sodium hydroxide to exit the electrode.
Platinum is considered the best catalyst for the complete 4-electron reduction of oxygen. However, due to its high cost, pure platinum is generally not used. Pure silver, carbon-supported silver, and carbon-supported platinum, among other catalysts, have been tried as less costly alternatives. A disadvantage of carbon-supported catalysts is that carbon may provide an energetically favorable path for the electrochemical reduction of oxygen to peroxide according to the following equation:O2+H2O+2e−→HO2−+OH−Peroxide is an unwanted impurity in the caustic stream, and its precipitation as sodium peroxide according to the following equation can cause liquid flow maintenance problems and damage the oxygen gas diffusion cathode:HO2−+2Na++OH−→Na2O2+H2O
Minimizing the generation of peroxide is of primary importance and highly desirable because peroxide can obstruct the cell operation and damage the oxygen diffusion cathode.
Therefore, an object of the present invention is to provide an oxygen-consuming chlor-alkali cell configured to minimize the formation of peroxide.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.